See my sibling comment that shows the math (I don't want to repost a full comment):

https://news.ycombinator.com/item?id=37975412

But to answer you as a landlord specifically, supply and demand will prevent that. There will be a one-time inflation event where you will raise your rents to a small degree, but you certainly can't pocket "all of it". Supply and demand will continue to exist just fine -- if you attempt to double your rent, everyone will simply move to a different landlord that raised prices by just 10%.

But poor tenants will be helped massively. (Again, see my linked comment for the math.)

Nothing. This is exactly was happened in France where there is welfare money for housing (APL): landlords raised prices saying that rented are helped anyway. In the end the situation is better for landlords, equals for people getting the welfare money (not everyone), and worse for people who don't.

More supply of housing would help in that case

It does get into elements of the basic problem though: i.e. would the government be (in the short/medium term) better off using that money to subsidize housing construction ?

I believe a UBI is our future, but I'm not entirely convinced we've actually hit the societal-scale productivity necessary to support it yet

Then we already know that our major grocery chains will do exactly the same thing, based on their current behavior. Unless UBI is handled with care the average recipient will just be a middle person between the government and the greedy.

People with basic income have time to take part in politics, to regulate away such monstrous landlords. Landlords already know this, and will pay to prevent it.

People that don’t want to work will suddenly become engaged in the political process? Sounds like work to me.

In addition to the people who don't want to work, there's the people who can't work (due to disabilities, family care responsibilities, discrimination in the workplace etc.), and the people who could get more done without a job in the private sector (volunteers, artists, and... politicians!).

The kinds of people who would benefit from UBI in this way are often the people least qualified or capable of engaging in politics. (Though in general, the credentialed class is full of people who don't understand the philosophical presuppositions either. The failure of education.)